Project Summary/Abstract Although a safe and efficacious preventative vaccine for HBV has been available for 30 years, it is not effective against chronic infection. The primary cause of chronic HBV (CHB) and the greatest impediment to developing a therapeutic vaccine is the direct and indirect effects of immune tolerance, primarily of CD4+ T cells, to HBV antigens. The resulting defective CD4+/CD8+ T cell response, poor cytokine production, insufficient neutralizing antibody (nAb) levels and poor response to conventional HBsAg vaccination characterize CHB infection. The objective of this revised SBIR Phase IIB continuation of R44 AI088919-06 proposal is to fully develop the use of virus-like particles (VLPs) to elicit nAb to prevent viral spread and prime CD4+/CD8+ T cells to eradicate intracellular HBV. For this purpose, in Phase I and II studies we have defined 8 neutralizing B cell epitopes from the HBV envelope PreS1 region, which were consolidated and inserted onto a species variant of the HBV core protein, namely the woodchuck hepatitis core antigen (WHc). PreS1 B cell epitopes were chosen because of their preferential expression on HBV virions as opposed to subviral particles. We chose the WHc as a vaccine carrier because the WHc and HBc are not cross-reactive at the B cell level and, just as importantly for our purposes, are only partially cross-reactive at the CD4+/CD8+ T cell level. Therefore, CD4+ T cells specific for WHc-unique T cell sites provide cognate T-B cell help for anti-PreS1 Ab production and are not curtailed by immune tolerance. In fact, in Phase II studies in HBc- and HBV-Tg mice, which are tolerant to HBc, immunization with hybrid PreS1-WHc VLPs elicited equivalent levels of high titer anti-PreS1 nAbs in wildtype and Tg mice. To determine the capacity of the anti-PreS1 nAbs to prevent an acute HBV infection and inhibit viral spread in an established infection, we showed that passive transfer of PreS1 nAbs into human-liver chimeric mice prevented acute infection and cleared serum HBV from mice previously infected with HBV (i.e., a model of CHB). At the T cell level, PreS1-WHc VLPs and WHc-based DNA immunogens elicited HBc-speciic CD4+ Th and CD8+ CTL responses. For the current proposal in Aim 1, we will select the final PreS1-WHc VLP candidate combination. To target intracellular HBV, in Aim 2 we will produce WHc-based DNA(RNA) constructs that circumvent immune tolerance and elicit CTL to the HBc and HBs. Expression of the HBc and HBs within the WHc VLP will allow WHc-specific (heterologous) CD4+ T cells to ?help? HBc/HBs-specific CTL thus bypassing impaired homologous T cell help as already demonstrated for PreS1-specific B cells. In Aim 3, the efficacy of VLP- DNA(RNA), prime-boost vaccine candidates will be evaluated in HBV-Tg mice and in an HBV-specific adeno- associated (AAV-HBV) infectious system, both models of immune tolerance and chronic HBV infection. In Aim 4, the WHc VLP production and purification will be optimized for transfer into a CMO or partner?s facility for scale up.